This invention relates in general to transmissions and, more particularly, to a traction drive transmission having at all critical surfaces pure rolling contact which is characterized by the absence of spinning.
Planetary transmissions typically rely on meshing gears to transfer power indeed, several planet gears between a sun gear and a ring gear. This arrangement distributes the load through the several planet gears, enabling the system to accommodate higher loads, and further provides several speed ratios, depending on where power is supplied to the system and where it is delivered from the system. But gear systems inherently contain backlash, are somewhat noisy, and set up vibrations. Moreover, the meshing gear teeth slide across each other, and the friction generated detracts from the efficiency of the system.
Planetary friction or traction drives, which rely solely on friction between rotating components to transfer power, do not have backlash and further operate with less noise and vibration than planetary gear systems, and in that sense they are superior to planetary gear systems. Furthermore, they have the capacity to operate at high velocity and with high efficiency. Even so, planetary systems of current design experience slipping in the form of spinning at critical surfaces where torque is transmitted, or to minimize this they may rely on narrow regions of contact where loads are heavily concentrated.
Actually, two types of slippage may exist along a line of contact between two critical surfaces in a traction drive transmission. One is characterized by pure sliding in which the difference in velocity between the two surfaces at the line of contact remains the same along the entire line of contact. Lubrication will accommodate this type of slippage, and indeed this type of slippage inherently exists when lubrication is present. With this type of slippage, the traction force along the line of contact increases as the slippage increases. Insofar as this discussion is concerned, this type of slippage constitutes pure rolling contact. The other type of slippage is characterized by spinning along the line of contact. Here the difference in velocity between the two critical surfaces varies along the line of contact. It may be in the same direction, in which case the pivot point is at or beyond the end of the line of contact, or it may be in two directions, in which event the pivot point is between the ends of the line of contact.
Spinning wastes energy and produces heat that may diminish the effectiveness of lubricants in the transmission. It also produces wear which limits the reliability of the transmission and likewise limits the torque transmitted by it. Excessive concentration of loads may cause early failure of the metal in the regions where the loads are concentrated and further reduces the capacity of the transmission to transmit torque.